NEWS & VIEWS

OPTOMECHANICS Push towards the quantum limit

Optomechanical set-ups use radiation pressure to manipulate macroscopic mechanical objects. Two experiments transfer this concept to the fields of superconducting microwave circuits and cold-atom physics.

Florian Marquardt In the set-up developed by Regal et al.9, a is in the Department of Physics, the Center for nanobeam is coupled capacitively to an on- NanoScience and the Arnold Sommerfeld Center for chip transmission-line microwave resonator Theoretical Physics, Ludwig-Maximilians-Universität made from superconducting material München, Theresienstr. 37, 80333 Munich, Germany. (see Fig. 1b), such that the motion of the e-mail: [email protected] beam modulates the microwave resonance frequency — much in the same way that ohannes Kepler might have been the the optical resonance is shi%ed by the !rst to realize that light can produce mechanical motion in the standard set-up J mechanical e"ects. In his treatise (Fig. 1a). $ere are several advantages to the De Cometis of 1619, Kepler suggested that approach explored by Regal and colleagues. the tail of a comet is de#ected by the Sun’s $e on-chip assembly is easily cooled by light. Today we know this is indeed true standard bulk refrigeration techniques, for the dust part of the tail, which points and in principle the size of the mechanical away from the Sun due to the radiation resonator is no longer constrained to be pressure. It took almost 300 years, however, larger than the wavelength of the radiation, until radiation-pressure e"ects were unlike in optical experiments that work demonstrated in the laboratory, a feat !rst Figure 1 Approaches to observing mechanical effects with re#ection. In a !rst demonstration achieved at the beginning of the last century of radiation. a, In the standard optomechanical set-up, of their set-up9, Regal et al. claim a near- by Pyotr N. Lebedev and, independently, by light (represented as a blue wave) inside an optical record force sensitivity and a displacement Ernest F. Nichols and Gordon F. Hull. cavity pushes against a mirror, mounted on a vibrating uncertainty a factor of 30 above the standard One route to exploiting the tiny forces cantilever (red). b, The set-up of Regal et al.9 uses a quantum limit. ($e quantum limit roughly that light exerts on matter is to have them act transmission-line microwave resonator instead of the corresponds to pinpointing the spatial on microscopic objects that are easy to push optical cavity, with a capacitively coupled nanobeam. coordinate down to the width of the ground around. $is has been done with great success c, In the set-up of Murch et al.10, the movable mirror state within one damping period, as dictated in the !eld of optical atom cooling and is replaced by a cloud of cold atoms, vibrating in an by the Heisenberg uncertainty principle.) trapping during the past twenty years. $e optical lattice potential (black). In work performed since then11, the same e"ects can be increased by many orders of group has also shown radiation-induced magnitude simply by introducing an optical cooling. One of the appealing prospects for cavity, where the light intensity is resonantly to the ground state of mechanical motion and such a set-up is to merge it with nonlinear enhanced. In such a set-up, photons bounce perform quantum-coherent experiments. circuit elements such as Josephson junctions around hundreds of thousands of times, each Such experiments would test quantum or qubits, which have already been coupled time transferring a small momentum kick mechanics in a new regime and provide to on-chip microwave resonators. $ese to the end-mirrors. If one of those mirrors is valuable insights into decoherence (and thus may a"ord sensitive measurements based on attached to a #exible mechanical element — a into the transition from the quantum regime nonlinear techniques and o"er a connection cantilever, for example — it will start to move to the classical world). Another, related, goal with quantum-information processing. under the e"ect of the light-induced force is to make ultrasensitive measurements, Murch et al.10 stick with optical (Fig. 1a). As soon as this happens, the light such as displacement detection or observing radiation, but they replace the mechanical intensity also changes, as the incoming quantum jumps between single vibrational element with a cloud of ultracold rubidium radiation is no longer in resonance with levels of a macroscopic object7. atoms. $e atoms are trapped inside a the optical mode. $e resulting coupled Two experiments reported in this !xed optical lattice made from a standing dynamics of light and matter can be exploited issue now add new twists to this theme, light wave in the cavity, where they vibrate to obtain both ampli!cation and cooling of and connect optomechanics to other independently in the troughs of the optical the mechanical motion. research !elds of current interest. Writing potential (see Fig. 1c). $e cloud of atoms Such cooling of macroscopic objects on page 555, Cindy Regal and co-workers9 acts as a tenuous dielectric medium, and has been demonstrated recently in a series show how to replace the optical cavity by therefore the atoms’ motion a"ects each of remarkable experiments1–7, giving rise to a microwave resonator on a chip. And on optical resonance frequency of the cavity. the !eld of optomechanics, which has joined page 561, Kater Murch and colleagues10 $e degree of these shi%s depends on nanoelectromechanical systems8 in the describe how the movable mirror can be the detailed placement of the atoms with race to cool a macroscopic object (such as a substituted by a cloud of cold atoms trapped respect to the intensity maxima of a given mirror with its many billions of atoms) down inside an optical lattice. optical mode. As a result, some properly nature physics | VOL 4 | JULY 2008 | www.nature.com/naturephysics 513 © 2008 Macmillan Publishers Limited. All rights reserved. NEWS & VIEWS chosen collective coordinate of the atomic towards a potentially fruitful interplay of References cloud replaces the position of the end- cold-atom physics with concepts known in 1. Höhberger-Metzger, C. & Karrai, K. Nature 432, 1002–1005 (2004). 2. Gigan, S. et al. Nature 444, 67–70 (2006). mirror in the standard set-up. One of the nano- and optomechanics. Exciting avenues 3. Arcizet, O., Cohadon, P. F., Briant, T., Pinard, M. & Heidmann, A. bene!ts provided by this scheme is that to be explored are the role of interatomic Nature 444, 71–74 (2006). there is no need to work hard to get to the interactions (giving rise to truly collective 4. Kleckner, D. & Bouwmeester, D. Nature 444, 75–78 (2006). 5. Schliesser, A., Del’Haye, P., Nooshi, N., Vahala, K. J. ground state. In fact, in the direction along modes) and the combination with atom- & Kippenberg, T. J. Phys. Rev. Lett. 97, 243905 (2006). the cavity axis, the vibrational motion of chip techniques. 6. Corbitt, T. et al. Phys. Rev. Lett. 98, 150802 (2007). the atoms is already in the ground state, Di"erent as the two approaches9,10 may 7. $ompson, J. D. et al. Nature 452, 72–75 (2008). 8. Schwab, K. C. & Roukes, M. L. Phys. Today 36–42 (July 2005). at microkelvin temperatures. $e atoms’ be, they illustrate the rapid pace of progress 9. Regal, C. A., Teufel, J. D. & Lehnert, K. W. Nature Phys. vibrations have a rather low quality factor, in the young !eld of optomechanics. 4, 555–560 (2008). but this is actually useful for observing Breakthroughs such as cooling of massive 10. Murch, K. W., Moore, K. L., Gupta, S. & Stamper-Kurn, D. M. Nature Phys. 4, 561–564 (2008). measurement backaction, the central result objects to the ground state might, therefore, 11. Teufel, J. D., Regal, C. A. & Lehnert, K. W. Preprint at of this work. $ese achievements point be just around the corner. (2008).

KAVLI PRIZE Science on all scales

How do you determine who does the each neurone is ‘connected’ to a thousand are indistinguishable from local stars, best scienti!c research? For Fred Kavli — others, and there are 100 billion such and indeed, from their odd radio signals, physicist, entrepreneur, philanthropist — neurones to coordinate. $e circuitry is they were considered to be ‘quasi-stellar’ the answer is clear: the most important mind-boggling, but nonetheless, research objects in our Galaxy only !%y years science is that which bene!ts has come a long way. of ago. However, of humanity. Established , $omas Jessell Caltech worked out that the spectrum in 2000, the Kavli of and of the quasar 3C273 only made sense Foundation of the Karolinska if it was moving away at 47,000 km s−1 focuses on a few Institute were awarded due to the expansion of the Universe. frontier areas of the prize Consequently, it must be emitting 1012 research that “for discoveries on the times more energy than the Sun. $e Kavli himself developmental and source of that power was eventually believes will functional identi!ed by Donald Lynden–Bell of make the largest logic of Cambridge, building on the hypothesis impact on our neuronal of Edwin Salpeter and Yakov Zeldovich quality of life. $e that quasars are powered by a central Foundation funds black hole. Lynden–Bell explained that endowed chairs as well the luminosity arose from frictional as !%een research institutes heating in the accretion disc surrounding at top universities. And on 28 May a black hole. He also predicted that 2008, the !rst recipients of the Kavli most massive galaxies harbour black prizes in astrophysics, nanoscience and holes, which has been veri!ed. neuroscience were announced. Schmidt and Lynden–Bell share On the smallest scale, the nanoscience the astrophysics prize for prize went to Louis E. Brus of Columbia having “dramatically expanded University and of Meijo the scale of the observable University “for their large impact in circuits”. Universe [that has] led to our the development of the nanoscience Rakic’s work present view of the violent !eld of the zero and one-dimensional on neuronal Universe in which massive nanostructures in physics, chemistry development FOUNDATION KAVLI black holes play a key role”. and biology”. Brus was working with and Jessell’s $e three biennial prizes, colloidal suspensions of semiconductor studies of neural jointly administered by the particles when he noticed that the optical circuitry have led to a Norwegian Academy of Science and properties depended on their size and framework for describing the Letters, Norwegian Ministry of Education shape. $ese ‘quantum dots’ behave as assembly of neural circuits within the brain. and Research, and the Kavli Foundation, arti!cial atoms with both fundamental Combined with the work of Grillner, mainly each consists of a scroll, a medal and and applied implications. Similarly, carbon on the subtleties of motor coordination of US$ 1 million. On 9 September 2008, the nanotubes exhibit interesting physics nerve cells, we have a clearer understanding seven winners will collect their awards at and are useful for applications owing of the relationship between the structure the inaugural ceremony in , . to their extraordinary strength. For his and behaviour of the networks within the As public outreach is an important aspect contribution to nanotube research, Iijima central nervous system. of the Kavli prizes, the activities will shares the nanoscience prize. Finally, at distances of billions of include public-awareness lectures as well Further up the length scale is a single light-years from Earth are quasars. as scienti!c symposia. nerve cell, or neurone. In the human brain, $rough a small optical telescope, they May Chiao

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